Testing apparatus is known, in which a tube to be tested rests on a number of turning rollers and is brought to rotation about its longitudinal axis by the rollers. In a carrier arrangement, two longitudinally movable transducer blocks are suspended and supported by the tube for running along a testing track in longitudinal direction of the tube. The transducers incorporated in the transducer blocks detect along helical curves of the surface of the rotating tube. In such an arrangement, it is disadvantageous that a pressing of the tube on the rollers be effected solely by the weight of the tube and the transducer blocks. If sufficient pressure is exerted by a transducer block, then an especially thin tube will bend appreciably, if the transducer block is located in the middle between two turning rollers. This results in strong radial movements and to an unsteady running of the tube. In U.S. Pat. No. 3,263,809, a testing device is described, in which a tube to be tested rests on turning rollers arranged transversely to the axis of the tube and is driven by those rollers in a helical movement, while the surface of the tube is scanned by stationary probes. Opposite to the turning rollers, devices for holding and pressing the tube onto the turning rollers are provided. Thus, they hold the tubes (which are frequently bent) in steady contact with the pressure rollers and prevent undesirable movements of the tube in radial directions. A uniform turning movement of the tube, being particularly desirable for electronic processing, is only guaranteed by a steady contact between the tube and turning rollers.
The utilization of devices for holding down the test specimen was either not possible up to now or only possible under great difficulties for the first described known testing apparatus, as stationary devices for holding the test specimen down would obstruct the longitudinal movement of the transducer blocks. The apparatus for holding down the test specimen, therefore, would have to be moved to the side, whenever a transducer block is to pass it, and this would have to be done very quickly, which would be difficult.
Furthermore, stationary, tiltable devices for holding down a test specimen located laterally to the specimen are not feasible even when the loading of the testing device is to be performed from the side. It is true that testing devices are known in which the scanning of a tube is performed from below by longitudinally moved transducers, and for which, thus, stationary devices on the upper side of the tube for holding down the tube are possible. These may be used, however, only for larger tube diameters, where there is enough space between two driving rollers for passage of a transducer block. Testing devices for relatively thin wall tubes and/or small diameter tubes cannot make use of the described scanning of the underside of the tube.